[TowerTalk] Homage to Ohmage

[KD7JYK DM09]

Peculiar.  I didn't start seeing 50 commonly until the 70's onward.  It 
really threw me, because much equipment, and documentation prior to that 
(probably going back another thirty years) was 52, and I was sweating it 
before I realized, it didn't matter.  Radios with 52 Ohms stamped next 
to the connector, filters, accessories, et cetera.

Does one suppose it depended on the industry?  Doing some research 
today, there's no end to the references, and debates of 50/52, so it's 
still somewhat of a big deal out there.

I no longer have direct access to JPL, but my library is filled with 
obscure documents, and reports collected over the decades.  My most 
recent acquisitions are, Potential End-To-End Imaging Information Rate 
Advantages of Various Alternative Communication Systems, Some Practical 
Universal Noiseless Coding Techniques, Reed-Soloman Encoders - 
Conventional vs Berlekamp's Architecture, Some Practical Noiseless 
Encoding Techniques Part II, and, Channel Coding And Data Compression 
System Considerations for Efficient Communication of Planetary Imaging 
Data, 1969 Flight Projects...  I used to grab everything, but now I 
focus on things I'm more likely to play with on my own or may provide 
insight to some of the items I have.

Along the lines of antennas/radio, I picked up NASA TN D-5081, 
Performance Characteristics Of The Apollo Astronaut Backpack Antenna. 
How this wound up in Israel is probably an interesting story I may never 
know.  No impedance is referenced.

Back to impedance...  I don't have a JPL lab here, per se, however, over 
the years, I grabbed whatever JPL, CalTech, Lear, McDonnell Douglas, 
Boeing, Shiley, places with locations, but no names, from Long Beach to 
Tehachapi, et cetera, didn't want.  Pallet, after pallet, after pallet 
of interesting, and peculiar items, some going back to day-one.  One 
interesting older seemingly radio related item was a 40kW rotary spark 
gap.  You'd think radio, but it was part of the oscillator for a carbon 
dating machine.

So, I dug out a few "50 Ohm" items.  This is particularly interesting:

Kay attenuator, model 300, spec'd at 50 Ohms, measures 52.4

Daven attenuator, RFB-451-50, spec'd at 50 Ohms, measures 52.0

DB Products load, spec'd at 50 Ohms, measures 51.3
DB Products load #2, specs at 50 Ohms, measures 51.4

Unknown model load, something aerospace related, 52.5

Unknown load, spec'd at 50 Ohms, measures 330.  Clearly has issues. 
Identical model, ~47.5, erratic, hard to get a fix, may also have issues.

Microlab attenuator, spec'd at 50, measures 53.2

Tossing out the two that may have issues, that's 52.13 Ohms average, 
which matches the old standard, regardless of labeling.

Now it gets more interesting.  I recall in the 70's, the wars over 50/52 
Ohms.  52 was the norm, 50 was really coming into play with consumer 
level items.  Why, exactly, from 52 to 50, I can only speculate.  I 
heard, ease of manufacturing, quality control, marketing, all sorts of 
things, but why an industry, precisely, shaved 2 Ohms after decades of 
manufacturing, and millions of products, and accessories, I never heard.

The 50/52 battle was over loss, and "getting out".  '52 Ohms resistance 
is more than 50, so that means more loss.  More loss in the cable means 
less power getting to the antenna.  NEVER use 52 Ohm cables or antennas 
with 50 Ohm radios.  You won't be heard well, if at all, 'cuz it messes 
with the SWR, and you may even fry your radio!'  So, if one has an only 
option of what to never use, but common, what becomes the new standard? 
  50, even if it's really the same old 52 stuff, just re-labeled, and 
close enough.

I pulled out three more loads.  These are Archer, made in the late 70's, 
or early 80's for consumer stuff, specifically, CB.  They measure 50.8, 
49.2, and 50.4, 50.13 average.  52.13 earlier, and 50.13 now?  Thinking 
my measuring device is off a little bit...  Still two quite distinctive 
numbers following two periods/levels of standards.  So 50 Ohms doesn't 
really enter the picture until we get about as far from Aerospace 
quality as we can, 70's-80's relatively cheapo-CB stuff, as now measured 
decades after the fact.

Interesting that your JPL items, and my various items are "50 Ohms". 
Mine are labeled one way, measure another.  Did you measure yours, or 
just read the label?  If they really are dead-on 50, I wonder what the 
application was, or if it just made the math easier to guesstimate.

Fortunately, cable-wise, 50, and 51 Ohm is common, as is 75.  They are 
either close enough to 50, or 52, or don't matter (75).  I've had a 
recent project that uses 92 Ohm (someone else's project, no shortcuts 
with oddball impedances), and a pile of 125 Ohm in storage.  I never 
found a use for the 125.

One last test.  Signal Corps, UG-529/U impedance matching unit made by 
O.E. Szekely.  53.4 Ohms on the BNC end, 122.6 on the twinax.  I suspect 
this is supposed to be 52, or 53 to 120, or 122, I forget the exact spec 
now, and it's not labeled further.  Either way, closer to 52, than 50. 
There's an example on ebay, with a designation of type 507-A, with 
different specs, 53 to 95.

Kurt